Apparatus and method for anodizing inner surface of tube

ABSTRACT

Provided is an apparatus for anodizing an internal surface of a tube including an electrolyte container storing an electrolyte solution, a first solution conduit connected with the electrolyte container to receive the electrolyte solution, a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit, a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube, a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit, and a cathode rod inserted from the second jig and extended to the first jig through the inner portion of the targeted tube, in which, while the electrolyte solution passes through the inner portion of the targeted tube, a cathode is applied to the cathode rod and an anode is applied to the targeted tube to perform an anodizing process.

TECHNICAL FIELD

The present invention relates to an apparatus and a method for anodizing a surface that is mainly composed of zirconium such as cladding of a fuel rod and has a non-planar surface.

BACKGROUND ART

Zirconium has a melting point of 1852° C. and very strong resistance to corrosion from the outside environment due to chemical stability. As a result, zirconium or an alloy thereof and an oxide thereof have been used in equipment which should not be corroded, such as fuel rod claddings of a nuclear power plant, stainless ceramic kitchen utensils, and tools used for handling chemicals.

Meanwhile, a nuclear fuel is covered by an aluminum or magnesium film so that toxic substances generated in a fission process are mixed with a coolant and not exposed to the outside. In the case of a light-water reactor, low enriched uranium dioxide (UO₂) powder is molded and sintered as cylindrical tablets having a diameter of 2 cm and a height of 2 cm to prepare dark brown pellets, and the pellets are put in a thin metal tube, a cladding, of about 3 mm made of a zirconium alloy (zircaloy) having good corrosion resistance to a high-temperature coolant and both ends are sealed.

Generally, tens to hundreds of fuel rods make a fuel assembly as one bundle and are used as one unit, and hundreds of fuel assemblies are include in a nuclear pile. The cladding is prepared with a wall thickness of 1 mm or less so that heat is conducted well, and during nuclear fission, a center temperature of the fuel pellets is about 200° C., a surface temperature is about 600° C., and temperatures of the inner surface and the surface of the fuel rod are about 400° C. and 300° C., respectively.

As such, when the fuel rod exposed at a high temperature is broken, a nuclear fission product is exposed to the outside to cause a serious problem, and as a result, efforts for increasing stability are continuously required.

As one index of stability applied to an apparatus for heating an operation fluid such as the kind, a critical heat flux (CHF) value is included, and the value means a maximum heat flux at which a heater is not broken but withstands in a heating situation. On the other hand, when the heat flux value is exceeded, it means that the heater is broken, and particularly, in a facility such as a nuclear power plant, a critical accident such as the core melting may be caused, and as a result, the CHF is a very important index.

All machines heating the operation fluid in addition to the nuclear power plant are managed by applying a safety factor of a predetermined level or more so as to not exceed the CHF value during the operation. However, since the safety management sacrifices performance of the device, researches for maintaining the safety factor and improving the performance by increasing the CHF value itself of the heater have been conducted. In studies in the past, it is known that as the surface of the heater is more hydrophilic, the CHF value is increased, and recently, furthermore, researches of largely increasing the hydrophilic property and the CHF by using nanoparticles or a minute structure have been conducted.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide an anodizing apparatus and a method thereof having an advantage of forming a minute structure on a surface so that an internal surface of a tube mainly composed of zirconium has a hydrophilic property.

Technical Solution

An exemplary embodiment of the present invention provides an apparatus for anodizing a surface of a tube, DeletedTexts including: an electrolyte container storing an electrolyte solution; a first solution conduit connected with the electrolyte container to receive the electrolyte solution; a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit; a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube; a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit; and a cathode rod inserted from the second jig and extended to the first jig through the inner portion of the targeted tube, in which while the electrolyte solution passes through the inner portion of the targeted tube, a cathode is applied to the cathode rod, and an anode is applied to the targeted tube to perform an anodizing process.

The apparatus may further include a cleaning solution container storing a cleaning solution, in which the cleaning solution container may be connected with the electrolyte container and the first solution conduit through a three-way valve.

The apparatus may further include a first constant-temperature water bath receiving a coolant, in which the electrolyte container and the cleaning solution container may be installed to be immersed in the coolant of the first constant-temperature water bath.

A flow meter and a flow control device may be installed in the first solution conduit to control a flow of the electrolyte solution or the cleaning solution passing through the first solution conduit.

The apparatus may further include a second constant-temperature water bath receiving a coolant, in which the targeted tube may be installed to be immersed in the coolant of the second constant-temperature water bath. In the first jig, a solution conduit connection hole to which the first solution conduit is insert-connected may be formed at a first side portion, and a targeted tube connection hole to which one end of the targeted tube is insert-connected may be formed at a second side portion.

The first jig may further include a cathode rod insertion hole to which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the first jig may be formed on a concentric axis.

In the second jig, a solution conduit connection hole to which the second solution conduit is insert-connected may be formed at a first side portion, and a targeted tube connection hole to which the other end of the targeted tube is insert-connected may be formed at a second side portion.

The second jig may further include a cathode rod insertion hole into which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the second jig may be formed on a concentric axis.

The first solution conduit, the targeted tube, and the second solution conduit may be connected with each other to form a U-shaped conduit, and the targeted tube may be positioned at a downstream branch of the U-shaped conduit.

The electrolyte solution may be a hydrofluoric acid solution, and the targeted tube may be a zirconium tube or a zirconium-alloy tube. In addition, the cathode rod may be a stainless steel rod.

Another exemplary embodiment of the present invention provides a method for anodizing an internal surface of a tube using an anodizing apparatus including an electrolyte container storing an electrolyte solution, a first solution conduit connected with the electrolyte container to receive the electrolyte solution, a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit, a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube, and a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit, the method including: supplying the electrolyte solution to flow through the first solution conduit, the targeted tube, and the second solution conduit; inserting a cathode rod to be extended to the first jig from the second jig through the inner portion of the targeted tube; and supplying power by applying a cathode to the cathode rod and applying an anode to the targeted tube, in which, while the electrolyte solution passes through the inner portion of the targeted tube, the anodizing process is performed.

The method may further include supplying a cleaning solution to flow through the first solution conduit, the targeted tube, and the second solution conduit after power supply stops and the anodizing process ends.

Advantageous Effects

As described above, according to the apparatus for anodizing an internal surface of a tube with zirconium or an alloy thereof, it is possible to form a hydrophilic oxide film with a minute structure formed on the internal surface of the tube which may not be easily applied in a method such as MEMS.

Further, it is possible to easily prepare a cylindrical internal surface such as a fuel rod with zirconium or an alloy thereof as a hydrophilic surface through the anodizing method. As the internal surface of the fuel rod is formed as the hydrophilic surface, a critical heat flux may be increased, and as a result, it is possible to increase efficiency of a nuclear power plant.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating fuel rods configuring a fuel assembly used in a nuclear power plant.

FIG. 2 is a schematic diagram illustrating a fuel assembly used in a nuclear power plant.

FIG. 3 is a configuration diagram schematically illustrating an apparatus for anodizing an internal surface of a cylindrical tube according to an exemplary embodiment of the present invention.

FIG. 4A is an exploded cross-sectional view illustrating a jig of the apparatus for anodizing the internal surface of the cylindrical tube according to an exemplary embodiment of the present invention, and FIG. 4B is a coupled cross-sectional view thereof.

FIG. 5 is a photograph illustrating a zirconium alloy (zircaloy) tube applied to the anodizing apparatus according to an exemplary embodiment of the present invention.

FIG. 6 is a SEM photograph of a minute structure formed on an internal surface of a zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention.

FIG. 7 is a photograph of a spreading phenomenon of droplets of water on the internal surface of the zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention.

FIG. 8 is a graph illustrating a critical heat flux enhancement ratio in a flow boiling situation in the case of using the zirconium alloy (zircaloy) tube prepared by the anodizing method according to another exemplary embodiment of the present invention.

MODE FOR INVENTION

In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. The drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.

According to previous studies (see Kandlikar, S. G., 2001, “A Theoretical Model to Predict Pool Boiling CHF Incorporating Effects of Contact Angle and Orientation,” Journal of Heat Transfer, Vol. 123, pp. 1071-1079), as a surface of a heater in a boiling apparatus is more hydrophilic, a critical heat flux (CHF) is increased.

When a CHF point is increased through surface modification, a heating engine may operate at a higher temperature, and as a result, energy efficiency is increased according to a principle of a Rankine cycle. Further, even though an actual operation temperature is not increased, a difference between a heat flux and the CHF during operation is increased, and as a result, stability is further increased by the difference. A cladding surface of the fuel rods exposed to a high temperature is prepared as a hydrophilic surface by applying the principle to the nuclear power plant to increase the CHF point, thereby ensuring higher stability.

FIG. 1 is a schematic diagram illustrating fuel rods configuring a fuel assembly used in a nuclear power plant, and FIG. 2 is a schematic diagram illustrating a fuel assembly used in a nuclear power plant.

Fuel rods 10 embedding uranium dioxide (UO₂) pellets (a sintered body) are arranged at 16×16 and fixed by spacer grids 21 and 23 to form a nuclear fuel assembly 20. Each fuel rod 10 has a cylindrical shape, and similarly, a plurality of pellets 12 having cylindrical shapes are accumulated therein, and in order to prevent the pellets 12 from moving in the fuel rod 10, a compression spring 15 may be inserted into the upper portion. A cladding 17 forming an outer skin of the fuel rod 10 may be made of a zirconium alloy which is an alloy mainly composed of zirconium, and is also called zircaloy.

Meanwhile, in order to make the surface of a solid hydrophilic, a method of changing a chemical characteristic of the surface and a method of changing a surface shape at a microscale or a nanoscale are used, and according to the exemplary embodiment of the present invention, the method of changing a surface shape at a microscale or a nanoscale is adopted.

That is, in the exemplary embodiment, while oxidation on the cladding surface is electrochemically promoted by applying the anodizing method to the cladding of the fuel rod, a micro/nanoscale structure may be formed on the surface. In the anodizing method, as compared with a microelectromechanical system (MEMS) process using a photolithography method in which a cleaning facility is basically required and thus basic investment costs are largely involved and it is difficult to be applied to a large area or a curved surface, there are advantages in that the anodizing method is cheap and may be applied to the large area or the curved surface.

In order to apply the anodizing method to the cladding surface of the fuel rod which has a tube shape, an anodizing apparatus which is different from a plate specimen is required. That is, for the anodization, basically, an anode and a cathode which face each other and an electrolyte filling a space therebetween are required. In addition, when a voltage is applied between the anode and the cathode, the anodization proceeds, and for a continuous anodizationDeletedTextsreaction, smooth circulation of the electrolyte is required.

FIG. 3 is a configuration diagram schematically illustrating an apparatus for anodizing an internal surface of a cylindrical tube according to an exemplary embodiment of the present invention. An anodizing apparatus for anodizing the internal surface of the tube as illustrated in FIG. 3 may be configured by satisfying the aforementioned condition.

Referring to FIG. 3, the anodizing apparatus according to the exemplary embodiment includes an electrolyte container 31, and a first solution conduit 37 and a second solution conduit 39 connected thereto. The electrolyte container 31 stores an electrolyte solution, and the first solution conduit 37 receives the electrolyte solution from the electrolyte container 31. The second solution conduit 39 is connected with the first solution conduit 37 with an anodization-targeted tube 50 therebetween, and the electrolyte solution supplied through the first solution conduit 37 flows into the targeted tube 50 to be discharged through the second solution conduit 39.

The electrolyte container 31 may be installed to supply the electrolyte solution to the anodization-targeted tube 50 by using positional energy without applying separate power.

The first solution conduit 37, the targeted tube 50, and the second solution conduit 39 are connected with each other to form a U-shaped conduit, and the targeted tube 50 may be positioned at a downstream (right) branch of the U-shaped conduit. Accordingly, even if the flow of the electrolyte solution stops, the electrolyte solution may be collected in the targeted tube 50, and the anodization may not stop but may last.

In order to fix one end of the targeted tube 50 to the downstream end of the first solution conduit 37, a first jig 41 is installed, and in order to the other end of the targeted tube 50 to an upstream end of the second solution conduit 39, a second jig 42 is installed. A structure of the jigs 41 and 42 will be described in detail with reference to FIGS. 4A and 4B.

Meanwhile, a cathode rod 45 serving as the cathode in the anodizing process is installed to be inserted from the second jig 42 and extended to the first jig 41 through the internal portion of the targeted tube 50. That is, while the electrolyte solution flows into the targeted tube 50, the anodizing process may be performed by applying the cathode to the cathode rod 45 and applying the anode to the targeted tube 50. A cathode terminal of a power supplier 47 that is separately provided is electrically connected with the cathode rod 45, and an anode terminal is electrically connected with the targeted tube 50 to apply the voltage. As the cathode rod 45, a conductive material rod having corrosion resistance to the electrolyte solution may be applied, and when the cathode rod 45 is bent in the targeted tube 50, a short circuit may be caused, and as a result, a bent material is not appropriate.

The power supplier 47 may perform real-time monitoring through a computer 49, and perform a function of inputting a pulse current or reversely applying the cathode and the anode by controlling the voltage and the current. According to a previous study (Chan Lee, Hyungmo Kim, Ho Seon Ahn, Moo Hwan Kim, and Joonwon Kim, “Micro/nanostructure Evolution of Zircaloy Surface Using Anodization Technique: Application to Nuclear Fuel Cladding Modification”, Applied Surface Science, Vol. 258, issue 22, pp. 8724-8731, 2012.09.01), in the anodization of a zirconium alloy, a surface structure varies according to a response duration, and as a result, properties of the surface including wettability are gradually changed. The power supplier 47 may monitor the current in real time, determine a reaction step based on the monitoring result, and easily selectively prepare a surface structure according to a desired purpose through a function of stopping the current supply. Further, when the power supplier 47 reversely applies the cathode and the anode, the anodizing process is changed into a plating process, and as a result, plating in the tube may be performed by using the apparatus of the exemplary embodiment.

The anodizing apparatus of the exemplary embodiment includes a cleaning solution container 32 storing a cleaning solution. The cleaning solution container 32 is connected with the electrolyte container 31 and the first solution conduit 37 through a three-way valve 35. The electrolyte solution may be supplied or the cleaning solution may be supplied through the first solution conduit 37 according to an operation of the three-way valve 35. During the anodizing process, the electrolyte solution is continuously supplied, and when the anodizing process is completed, the cleaning solution is immediately supplied. The cleaning solution may be, for example, deionized water.

The anodizing apparatus of the exemplary embodiment includes a first constant-temperature water bath 51 which receives a coolant, and the electrolyte container 31 and the cleaning solution container 32 may be installed to be immersed in the coolant of the constant-temperature water bath 51.

Further, a flow meter and a flow control device 36 are installed in the first solution conduit 37 to control the flow of the electrolyte solution or the cleaning solution passing through the first solution conduit 37. As a result, a reaction speed of the anodization influenced by the flow or uniformity of the formed structure may be controlled.

The anodizing apparatus of the exemplary embodiment includes a second constant-temperature water bath 61 receiving the coolant. The targeted tube 50 is installed to be immersed in the coolant received in the second constant-temperature water bath 61 to help a proper temperature be maintained. The coolant is circulated by a circulation device 63 connected with the second constant-temperature water bath 61 to maintain the temperature. The coolant may be, for example, water. Since the second constant-temperature water bath 61 does not have a closed structure in which the coolant completely surrounds the outside of the targeted tube 50 but has an opened structure in which an upper portion is opened, the targeted tube 50 may be easily inserted and ejected before and after an experiment. During the anodization, heat is generated due to the reaction, and because the temperature may be a factor that has a large effect on the anodization, proper temperature control is required for an optimum reaction condition.

In the exemplary embodiment, a temperature control device is provided in the circulation device 63 to control the temperature of the coolant supplied to the first constant-temperature water bath 51 and the second constant-temperature water bath 61. Accordingly, the temperature control effect may be maximized by simultaneously performing indirect cooling through the heat transfer from the outer surface of the targeted tube 50 by using the second constant-temperature water bath 61 and cooling through temperature control of the electrolyte solution itself directly contacting a reaction surface by using the first constant-temperature water bath 51.

The electrolyte solution discharged through the second solution conduit 39 is collected in an electrolyte receiving bath 65, and the collected electrolyte solution is again circulated to the electrolyte container 31 to be reused.

FIG. 4A is an exploded cross-sectional view illustrating a jig of the apparatus for anodizing the internal surface of the cylindrical tube according to an exemplary embodiment of the present invention, and FIG. 4B is a coupled cross-sectional view thereof. FIGS. 4A and 4B illustrate the second jig 42 illustrated in FIG. 3, but since the structure of the first jig 41 is similar to the structure of the second jig 42 except for only a formation direction of an opening, hereinafter, only the second jig 42 will be described.

Referring to FIG. 4A, the second jig 42 includes a jig body 421 in which a solution conduit connection hole 421 a to which the second solution conduit 39 is insert-connected is formed at a first side portion, and a targeted tube connection hole 421 b to which the other end of the targeted tube 50 is insert-connected is formed at a second side portion. An upper cover 425 is coupled with an upper portion of the jig body 421 by a bolt 424, and a lower cover 423 is coupled with a lower portion of the jig body 421 by a bolt 424. A cathode rod insertion hole 421 c into which the cathode rod 45 is inserted is formed at an upper end of the jig body 421, and the targeted tube connection hole 421 b and the cathode rod insertion hole 421 c are formed on a concentric axis. At the portion where the cathode rod insertion hole 421 c and the targeted tube connection hole 421 b are formed, O-rings 427 and 428 are installed between the jig body 421 and the upper cover 425 and between the jig body 421 and the lower cover 423 to prevent the electrolyte solution from leaking to the outside. Threads engaging with each other are formed on an outer surface of the end of the second solution conduit 39 and an internal surface of the solution conduit connection hole 421 a to be screw-coupled with each other.

Referring to FIG. 4B, the second solution conduit 39, the targeted tube 50, and the cathode rod 45 are connected to the coupled second jig 42. That is, the second solution conduit 39 is insert-connected to the solution conduit connection hole 421 a, the targeted tube 50 is insert-connected to the targeted tube connection hole 421 b, and the cathode rod 45 is inserted into the cathode rod connection hole 421 c. The inserted cathode rod 45 has a concentric axis with the targeted tube 50 and is installed to pass through the inner portion of the targeted tube 50.

In the anodizing apparatus according to the exemplary embodiment, for example, the targeted tube 50 may be a zircaloy tube, and the electrolyte solution may be a hydrofluoric acid solution. The cathode rod 45 inserted into the targeted tube 50 may be a stainless steel rod, and the first solution conduit 37 and the second solution conduit 39 may be formed of perfluoroalkoxy materials. In addition, the jig body 421, the upper cover 425, the lower cover 423, and the bolt 424 configuring the second jig 42 may be made of polytetrafluoroethylene (PTFE) materials, and the O-rings 427 and 428 may be Viton® O-rings.

<Anodizing Method>

A method for anodizing the internal surface of the tube prepared by zirconium or an alloy thereof according to the exemplary embodiment of the present invention by using the anodizing apparatus illustrated in FIG. 3 will be described below.

First, the electrolyte solution stored in the electrolyte container 31 is supplied to flow through the first solution conduit 37, the targeted tube 50, and the second solution conduit 39. In the exemplary embodiment, a solution of which the temperature of the electrolyte solution is in a range 0 to 15° C. may be applied, and a solution of which the concentration of the electrolyte solution is in a range of 0.01 to 1 wt % may be applied, and for example, as the electrolyte solution, a hydrofluoric acid solution of 0.5 wt % at 10° C. or less may be applied. As the targeted tube 50, a tube of which the length is 10 cm or more may be applied, and a zirconium-alloy tube of which an outer diameter is ⅜ of an inch may be applied. In addition, a flow velocity of the electrolyte solution flowing in the targeted tube 50 is about 300 to 1000 ml/min.

Next, the cathode rod 45 is inserted to be extended to the first jig 41 from the second jig 42 through the inner portion of the targeted tube 50. The cathode rod 45 may be pre-inserted before the electrolyte solution is supplied. In the exemplary embodiment, as the cathode rod 45, a stainless steel rod having an outer diameter of 3 mm may be applied.

Next, power may be supplied by applying the cathode to the cathode rod 45 and applying the anode to the targeted tube 50. In the exemplary embodiment, a voltage of 5 to 40 V may be applied between the anode and the cathode, and for example, a voltage of 15 V may be applied. The voltage may be applied for a time of 10 to 40 minutes.

Through such a process, while the electrolyte solution flows into the targeted tube 50, the anodizing process is performed, and as a result, an uneven structure having a micro/nanoscale is formed on the inner surface of the targeted tube 50. The inner surface of the targeted tube 50 has hydrophilicity due to the formed uneven structure.

Next, after the anodizing process ends by stopping the power supply, the cleaning solution is supplied to flow through the first solution conduit 37, the targeted tube 50, and the second solution conduit 39. After the anodizing process ends (the power supply stops), a contact with the electrolyte solution (hydrofluoric acid solution) has a negative influence on the structure such that a minute structure of the surface may be rapidly broken, and therefore, the electrolyte solution needs to be rapidly cleaned. To this end, the cleaning solution may be injected immediately after the reaction ends by operating the three-way valve 35.

Experimental Example

As described above, the anodizing method was performed by using the anodizing apparatus according to the exemplary embodiment of the present invention. As the electrolyte solution, a 0.5 wt % hydrofluoric acid solution was used, and as the cleaning solution, deionized water was used.

FIG. 5 is a photograph illustrating a zirconium alloy (zircaloy) tube applied to the anodizing apparatus according to an exemplary embodiment of the present invention. The zircaloy tube is made of a zirconium alloy having an outer diameter of ⅜ of an inch and a length of 53 cm, and is illustrated to be cut in a length direction so as to show the inner portion.

As the cathode rod, the stainless steel rod with an outer diameter of 3 mm is applied, and the voltage applied between the anode and the cathode is 15 V, while a reaction time is 25 minutes. The temperature of the constant-temperature water bath was kept at 3° C.

FIG. 6 is an SEM photograph of a minute structure formed on an internal surface of a zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. According to the anodizing method using the anodizing apparatus described above, it is shown that the minute structure may be actually formed on the inner surface of the large-area curved surface.

FIG. 7 is a photograph of a spreading phenomenon of droplets of water on the internal surface of the zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. That is, since it was observed that droplets of water placed on the surface spread within rapidly, it can be seen that the surface formed by the anodizing method according to the exemplary embodiment has a very high hydrophilic property, and accordingly, as described above, improvement of a high critical heat flux may be expected.

According to the exemplary embodiment, an experiment for checking a critical heat flux enhancement ratio under a flow boiling condition was performed by using the zircaloy tube with the minute structure formed at the internal surface. That is, when water flows into the tube and heat is applied from the outside to increase the temperature, the flowing water reaches the critical heat flux around a predetermined point of the tube. A thermocouple is mounted on the zircaloy tube in which the minute structure is formed on the internal surface to measure a critical heat flux (CHF) value, and the degree of enhancement of the CHF is illustrated in FIG. 8.

In FIG. 8, a mass flux means a flux of water flowing in the tube, and an inlet temperature means a temperature of water flowing into the tube. Referring to FIG. 8, about a 60% increase is shown under a condition of a mass flux of 1500 kg/m²s. The result verifies that the surface minute structure made under the above condition significantly increases the CHF in an actual flow boiling condition.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. An apparatus for anodizing an internal surface of a tube, comprising: an electrolyte container storing an electrolyte solution; a first solution conduit connected with the electrolyte container to receive the electrolyte solution; a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit; a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube; a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit; and a cathode rod inserted from the second jig and extended to the first jig through the inner portion of the targeted tube, wherein while the electrolyte solution passes through the inner portion of the targeted tube, a cathode is applied to the cathode rod, and an anode is applied to the targeted tube to perform an anodizing process.
 2. The apparatus of claim 1, further comprising a cleaning solution container storing a cleaning solution, wherein the cleaning solution container is connected with the electrolyte container and the first solution conduit through a three-way valve.
 3. The apparatus of claim 2, further comprising a first constant-temperature water bath receiving a coolant, wherein the electrolyte container and the cleaning solution container are installed to be immersed in the coolant of the first constant-temperature water bath.
 4. The apparatus of claim 2, wherein a flow meter and a flow control device are installed in the first solution conduit to control a flow of the electrolyte solution or the cleaning solution passing through the first solution conduit.
 5. The apparatus of claim 1, further comprising a second constant-temperature water bath receiving a coolant, wherein the targeted tube is installed to be immersed in the coolant of the second constant-temperature water bath.
 6. The apparatus of claim 1, wherein in the first jig, a solution conduit connection hole to which the first solution conduit is insert-connected is formed at a first side portion, and a targeted tube connection hole to which one end of the targeted tube is insert-connected is formed at a second side portion.
 7. The apparatus of claim 6, wherein the first jig further includes a cathode rod insertion hole to which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the first jig are formed on a concentric axis.
 8. The apparatus of claim 1, wherein in the second jig, a solution conduit connection hole to which the second solution conduit is insert-connected is formed at a first side portion, and a targeted tube connection hole to which the other end of the targeted tube is insert-connected is formed at a second side portion.
 9. The apparatus of claim 8, wherein the second jig further includes a cathode rod insertion hole into which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the second jig are formed on a concentric axis.
 10. The apparatus of claim 1, wherein a cooler or a heater is included to control a temperature of the coolant, and a temperature control device that maintains a predetermined temperature during an anodizing reaction by contacting the targeted tube and the coolant is further included.
 11. The apparatus of claim 1, wherein the first solution conduit, the targeted tube, and the second solution conduit are connected with each other to form a U-shaped conduit, and the targeted tube is positioned at a downstream branch of the U-shaped conduit.
 12. The apparatus of claim 1, wherein the electrolyte solution is a hydrofluoric acid solution.
 13. The apparatus of claim 1, wherein the targeted tube is a zirconium tube or a zirconium-alloy tube.
 14. The apparatus of claim 1, wherein the cathode rod is a stainless steel rod.
 15. A method for anodizing an internal surface of a tube using an anodizing apparatus including an electrolyte container storing an electrolyte solution, a first solution conduit connected with the electrolyte container to receive the electrolyte solution, a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit, a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube, and a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit, the method comprising: supplying the electrolyte solution to flow through the first solution conduit, the targeted tube, and the second solution conduit; inserting a cathode rod to be extended to the first jig from the second jig through the inner portion of the targeted tube; and supplying power by applying a cathode to the cathode rod and applying an anode to the targeted tube, wherein while the electrolyte solution passes through the inner portion of the targeted tube, an anodizing process is performed.
 16. The method of claim 15, further comprising supplying a cleaning solution to flow through the first solution conduit, the targeted tube, and the second solution conduit after power supply stops and the anodizing process ends. 